In 3GPP LTE (3rd Generation Partnership Project Long Term Evolution), OFDMA (orthogonal frequency division multiple access) is employed as a mode of downlink communication from a base station (also called “eNB”) to a terminal (also called “user equipment”) and SC-FDMA (single-carrier frequency-division multiple access) is employed as a mode of uplink communication from a terminal to a base station (see, for example, NPL 1 to NPL 3).
In LTE, a base station performs communication by allocating resource blocks (RBs) within a system band to a terminal for each time unit called subframe. Further, the base station transmits, through a downlink control channel (PDCCH: Physical Downlink Control Channel), control information according to which the terminal transmits and receives data. Further, in LTE Release 11, the base station can also transmit the control information through the EPDCCH (Enhanced PDCCH), i.e. an enhancement of the PDCCH. The terminal decodes the control information transmitted thereto by the received PDCCH signal or EPDCCH signal and obtains information pertaining to frequency allocation or adaptive control that is needed for the transmission and reception of data.
Further, in LTE, HARQ (hybrid automatic repeat request) is applied to downlink data. That is, the terminal feeds back, to the base station, a response signal representing a result of detection of an error in downlink data. The terminal performs a CRC (Cyclic Redundancy Check) on the downlink data. If an operation result of the CRC contains no error, the terminal feeds back an acknowledgement (ACK) as the response signal to the base station. If the operation result of the CRC contains an error, the terminal feeds back a negative acknowledgement (NACK) as the response signal to the base station. This response signal (i.e. an ACK/NACK signal) is fed back through an uplink control channel such as the PUCCH (Physical Uplink Control Channel).
In LTE, a plurality of ACK/NACK signals that are transmitted from a plurality of terminals, as shown in FIG. 1, are spread by a ZAC (zero auto-correlation) sequence having zero auto-correlation characteristics on a time axis (multiplied by the ZAC sequence) and code-multiplexed within the PUCCH. In FIG. 1, (W(0), W(1), W(2), W(3)) represents a Walsh sequence having a sequence length of 4 and (F(0), F(1), F(2)) represents a DFT (discrete Fourier transform) sequence having a sequence length of 3.
As shown in FIG. 1, in the terminals, the ACK/NACK signals are first subjected to primary spreading into frequency components each corresponding to one SC-FDMA symbol by the ZAC sequence (sequence length of 12). That is, the ZAC sequence having a sequence length of 12 is multiplied by ACK/NACK signal components represented by complex numbers. Next, the ACK/NACK signals subjected to primary spreading and the ZAC sequence serving as reference signals are subjected to secondary spreading by the Walsh sequence (sequence length of 4: W(0) to W(3)) and the DFT sequence (sequence length of 3: F(0) to F(2)), respectively. That is, the respective components of the signals having a sequence length of 12 (the ACK/NACK signals subjected to primary spreading or the ZAC sequence serving as reference signals) are multiplied by each component of an orthogonal code sequence (OCC: orthogonal cover code, the Walsh sequence, or the DFT sequence). Furthermore, the signals subjected to secondary spreading are converted into signals having a sequence length of 12 on the time axis by inverse discrete Fourier transform (IDFT or IFFT (inverse fast Fourier transform)). Moreover, a cyclic prefix (CP) is appended to each of the signals subjected to IFFT. This forms a one-slot signal composed of seven SC-FDMA symbols.
Further, as shown in FIG. 2, the PUCCH is allocated to each terminal by subframe unit.
ACK/NACK signals from different terminals are spread (multiplied) using a ZAC sequence defined by different cyclic shift amounts (cyclic shift indices) or an orthogonal code sequence corresponding to different sequence numbers (OC indices: orthogonal cover indices). The orthogonal code sequence is a set of a Walsh sequence and a DFT sequence. Further, the orthogonal code sequence is also referred to as “block-wise spreading code sequence”. Therefore, by using conventional de-spreading and correlation processing, the base station can demultiplex a plurality of these code-multiplexed ACK/NACK signals (see, for example, NPL 4). It should be noted that since there is a limit to the number of ACK/NACK signals that can be code-multiplexed or cyclic-shift-multiplexed per frequency resource block (RB), an increase in the number of terminals causes the ACK/NACK signals to be frequency-multiplexed onto different RBs. Code and frequency resources through which ACK/NACK signals are transmitted are hereinafter called “PUCCH resources”. The number (index) of a PUCCH resource is determined by the RB number of an RB through which to transmit an ACK/NACK signal and an OC index and a cyclic shift amount in the RB. Since multiplexing of a ZAC sequence by cyclic shifts can be deemed as a type of code multiplexing, orthogonal codes and cyclic shifts are hereinafter sometimes collectively referred to as “codes”.
In LTE, allocation based on PDCCH or EPDCCH control information mapping results is employed as a method for identifying a PUCCH resource through which to transmit an ACK/NACK signal.
In the case of the PDCCH, the control information is not mapped to the same resources between a plurality of terminals, whereby PDCCH resources and PUCCH resources are associated in one-to-one correspondence with each other. The PDCCH is constituted by one or more L1/L2 CCHs (L1/L2 Control Channels). Each L1/L2 CCH is constituted by one or more CCEs (Control Channel Elements). That is, a CCE is a fundamental unit by which the control information is mapped to the PDCCH. Further, in a case where one L1/L2 CCH is constituted by a plurality of (two, four, or eight) CCEs, the L1/L2 CCH is allocated a plurality of contiguous CCEs beginning at a CCE having an even-numbered index. In accordance with the number of CCEs that are needed for the indication of the control information to a resource allocation target terminal, the base station allocates the L1/L2 CCH to the resource allocation target terminal. Then, the base station maps the control information to a physical resource corresponding to a CCE of this L1/L2 CCH and transmits the control information. Note also here that each CCE is associated in one-to-one correspondence with a PUCCH resource through which to transmit an ACK/NACK signal. Therefore, a terminal having received the L1/L2 CCH identifies a PUCCH resource corresponding to a CCE constituting this L1/L2 CCH and transmits an ACK/NACK signal to the base station through this resource (i.e. a code and a frequency). Note, however, that in a case where the L1/L2 CCH occupies a plurality of contiguous CCEs, the terminal transmits the ACK/NACK signal to the base station through a PUCCH resource corresponding to a CCE having the smallest index of a plurality of PUCCH configuration resources respectively corresponding to the plurality of CCEs. Specifically, the PUCCH resource number nPUCCH is determined according to the following formula (Math. 1) (see, for example, NPL 3).nPUCCH=nCCE+NPUCCH  [Math. 1]
Note here that nPUCCH is the PUCCH resource number of the PUCCH resource through which to transmit the ACK/NACK signal. NPUCCH denotes a PUCCH resource offset value that is commonly given within a cell, and nCCE denotes the CCE number of a CCE having the smallest index of the CCEs to which the PDCCH for the terminal is mapped.
In the case of the EPDCCH, a resource for an ACK/NACK signal with respect to a downlink data channel (PDSCH: Physical Downlink Shared Channel) allocated by the EPDCCH is determined by using a PUCCH resource offset that is given from a higher layer for each EPDCCH set and an index of enhanced control channel elements (ECCEs: Enhanced CCEs) serving as element units that constitute each EPDCCH. That is, a PUCCH resource number corresponding to the EPDCCH is determined by using the value of the PUCCH resource offset and the ECCE number of an ECCE having the smallest index of the ECCEs to which the EPDCCH is mapped. Appropriate setting of PUCCH resource offsets corresponding to each separate EPDCCH makes it possible, even in an environment where the PDCCH and one or more EPDCCH sets are used, to appropriately allocate the ACK/NACK signal that the terminal transmits.
While making PUCCH resource offsets take on sufficiently great values makes such operation possible that there is no overlap of PUCCH regions corresponding to a plurality of EPDCCH sets, the total amount of PUCCH resources to be secured increases with the number of EPDCCH sets to be used, with the result that there is an increase in PUCCH overhead.
On the contrary, adjusting PUCCH resource offsets makes such operation possible that a plurality of PUCCH regions overlap. In this case, the total amount of PUCCH resources to be secured can be reduced. Note, however, that a collision of PUCCH resources to be used may occur between EPDCCH sets whose PUCCH regions overlap. In a case where such a collision of PUCCH resources occurs, there is deterioration in downlink throughput, as only one of the EPDCCH sets can be allocated. To address this problem, a control bit called ARO (ACK/NACK resource offset) by which a further offset is indicated is added into EPDCCH control information as a method for avoiding a collision of PUCCH resources while using a plurality of overlapping PUCCH regions. Specifically, a PUCCH resource is determined according to the following formula (Math. 2) (see, for example, NPL 3).nPUCCH,EDPCCH=nECCE(n)+ΔARO+NEPDCCH(n)  [Math. 2]
Note here that nPUCCH,EPDCCH is the PUCCH resource number. NEPDCCH(n) is the PUCCH resource offset corresponding to the nth EPDCCH set(n), ΔARO is an offset value, and nECCE(n) is, of the ECCE numbers defined in the EPDCCH set(n), the ECCE number of an ECCE having the smallest index of the ECCEs actually used for the transmission of the EPDCCH. It should be noted that NEPDCCH(n) is a value that is indicated by a UE-specific higher layer.
Incidentally, M2M (machine-to-machine) communication, which achieves services through autonomous communication between machines without users' judgments, has recently been expected as a mechanism that underpins future information societies. Specific cases to which the M2M system is applied include smart grids. A smart grid is an infrastructure system that efficiently supplies a lifeline such as electricity or gas, and autonomously and effectively adjusts demand balance of resources by performing M2M communication between smart meters installed in households or buildings and a central server. Other cases to which the M2M communication system is applied include stock control, a monitoring system for environmental sensing or telemedicine, remote management of stock or charge of self-vending machines, and the like.
In the M2M communication system, attention has been focused, in particular, on the utilization of a cellular system having a wide communication area. In 3GPP, central network upgrading for M2M, called machine-type communication (MTC), is being standardized (see, for example, NPL 5) under LTE and LTE-Advanced standardization, and specifications are being discussed with reduction in cost of terminals, reduction in power consumption, and coverage enhancement as requirements.
In order to achieve reduction in cost of terminals, an LTE Release 13 MTC-compatible terminal (hereinafter sometimes referred to as “MTC terminal”) supports only a frequency bandwidth of 1.4 MHz (hereinafter sometimes referred to as “MTC narrow band”). Further, in MTC coverage enhancement for further enlargement of the communication area, a repetition technique is employed by which to enhance coverage by causing received signal power to be improved by repeatedly transmitting the same signal more than once and combining these signals on the receiving side.